In the second variant the invention further relates to a device for generating control data which are adapted to control a laser treatment device for surgical correction of defective vision of an eye of a patient, wherein a correction surface is predetermined which is to be produced as cut surface in the cornea for the removal of a volume and which is non-rotation-symmetrical relative to a main direction of incidence, and wherein the device generates the control data on basis of the correction surface such that, during operation, the laser treatment device produces the correction surface in the cornea and that, during generating of the control data, the device adapts the non-rotation-symmetrical correction surface to a contour that is circular when viewed in the main direction of incidence of the laser radiation.
Spectacles are the traditional way of correcting defective vision in the human eye. However, refractive surgery which corrects defective vision by altering the cornea is now also increasingly being used. The aim of the surgical methods is to selectively alter the cornea so as to influence refraction. Differing procedures of surgeries are known for this purpose. Currently the most widespread is the so-called laser-assisted in situ keratomileusis, also abbreviated to LASIK. Firstly, a lamella of the cornea is cut on one side from the cornea surface and folded to the side. This lamella can be cut by means of a mechanical microkeratome or also by means of a so-called laser keratome, such as is marketed e.g. by Intralase Corp., Irvine, USA. After the lamella has been cut and folded to the side, the LASIK operation uses an excimer laser, which removes the thus-exposed corneal tissue by ablation. After volume in the cornea has been vaporized in this manner the lamella of the cornea is folded back into its original place.
The use of a laser keratome to expose the lamella is advantageous as the danger of infection is thereby reduced and the cut quality increased. In particular the lamella can be produced with a very much more consistent thickness. The cut is also potentially smoother, which reduces sight problems due to this boundary surface which remains even after the operation. To produce the cut, a series of incisions of the eye are made at predetermined points such that the cut surface is formed as a result. With the laser keratome the cut surface forms the lamella to be folded back before the use of laser ablation.
With the conventional LASIK method exposed corneal tissue is vaporized, which is also called “grinding” of the cornea by means of laser radiation. The volume removal which is necessary to correct defective vision is set for each surface element of the exposed cornea by the number of laser pulses and their energy. Therefore, in the LASIK method, a so-called shot file is provided for the ablation laser which defines, for different points on the cornea, how often, and with what energy, the laser beam is to be directed onto defined points on the cornea. The volume removal is heuristically determined, not least because it depends greatly on the ablation effect of the laser beam, therefore on the wavelength, fluence etc. of the radiation used. The state of the cornea also plays a role; in particular the moisture content of the cornea is to be mentioned here. WO 96/11655 describes a device and a process for the LASIK method. In particular a formula is given which calculates the radius of curvature to be achieved from the pre-operative radius of curvature of the cornea and the desired diopter correction. A similar calculation is described in EP 1153584 A1—also for corneal ablation by means of LASIK.
U.S. Pat. No. 5,993,438 proposes the removal of a volume from the cornea by vaporization and absorption in the cornea.
WO 2005/092172 discloses how optical refraction power measurements which have been determined in one plane can be transferred into another plane. The document mentions that this process can be used for different eye treatments, in particular for laser-supported ablation.
A further laser-based eye surgery method is not to vaporize the volume to be removed from the cornea, but to isolate it by a laser cut. The volume is thus no longer ablated, but isolated in the cornea by a three-dimensional cut surface and thus made removable. Empirical values which have been developed for grinding the cornea by means of ablation laser radiation cannot be used for such methods. Instead, control data are required to operate the laser for isolating the volume to be removed from the cornea. One such procedure for eye surgery is described in U.S. Pat. No. 6,110,166 and U.S. Pat. No. 7,131,968. Different volume forms are shown in U.S. Pat. No. 6,110,166 and it is mentioned that the proper volume can be chosen by a person skilled in the art.
DE 102006053118 A1 describes the production of control data for the volume-isolating correction of defective vision.
It is known from DE 102006053120 A1 and DE 102006053119 A1 from Carl Zeiss Meditec AG to base the production of such defective vision on data which give the optical refraction power of spectacles suitable for correcting defective vision. It is also known from this published document, which thus describes a method of the mentioned type and a device of the mentioned type, to use data which also bring about a correction of an astigmatism or corrections of higher-order aberrations. By using data for defective vision which are intended for a conventional spectacle correction, the approach known from DE 102006053120 A1 achieves a considerable simplification in pre-operative eye measurement, as the production of spectacle correction data is daily practice in ophthalmology. However, this simplification also means a degree of limitation of the possible correction results, because inevitably only corrections which would also be possible with normal spectacles can be achieved. It is also to be taken into account here that corrections such as are possible e.g. with varifocals are ruled out for the approach according to DE 102006053120 A1 as such corrections always assume that, depending on the viewing direction, the axis of vision passes through the spectacle lens at different points, which makes it possible to be able to bring different optical properties of the spectacles to bear for different viewing directions (e.g. reading directed more downwards, or viewing directed more into the distance). This does not apply in the case of refractive surgery on the cornea because movement of the eye obviously causes the cornea to move as well when the direction of viewing changes. Thus, unlike with a spectacle lens, there is no change in the point where the optical axis penetrates the cornea when the eyeball rotates. The approach known from DE 102006053120 A1 can thus consequently use only comparatively simple spectacle defective-vision correction data as an input variable for control data, with the consequence of correspondingly limited possibilities of correction.
It is known from DE 10334110 A1 from Carl Zeiss Meditec AG to produce a cut surface which at least partly circumscribes the volume to be removed in order to correct defective vision by shifting the focus of the laser radiation along orbits following contour lines or along a spiral which is based on such contour lines. The planes in which the contour lines are defined or on the basis of which the spiral is defined are oriented perpendicular to the main direction of incidence of the treatment laser radiation. Shifting the focus along the optical axis, which is customarily undertaken by an adjustable zoom lens or similar, thus has the smallest possible restriction on the speed of shifting along the path. As this shift of the focus is generally much slower than the deflection across the main direction of incidence of the treatment laser radiation, the result is a rapid production of the cut surface.
This publication describes that corrections of defective vision which go beyond a spherical correction, for example to correct an astigmatism, consistently require aspherical cut surfaces, for example cut surfaces in the form of an ellipsoid. In this connection DE 10334110 A1 describes that such a cut surface can be given a circular contour as seen along the main direction of incidence if the operating laser radiation is deactivated in sections which go beyond such a circular contour. FIG. 11 shows the conditions obtained in this case. A sectional representation through a cornea 5 in which a volume 18 is isolated and prepared for removal is shown. The volume 18 is defined by an anterior cut surface (flap surface 19) produced substantially parallel to the cornea front surface and a posterior cut surface (lenticle surface 20). A top view 33 of the lenticle surface 20 is shown at the bottom of FIG. 11. It determines the curvature the front of the cornea 15 has once the volume 18 is removed. FIG. 11 shows a case in which an astigmatic correction is to be undertaken, which is why the lenticle surface 20 is an ellipsoid. At the top of FIG. 11, therefore, two cut lines 20.1 and 20.2 which correspond to the main axes H1 and H2 of the ellipsoid surface, are shown for the cut surface 20. In the top view 33 the volume 18 has a circular contour. Furthermore, the ellipsoidal lenticle surface 20 is produced by a spiral-shaped path 32 along which the position of the focus of the treatment laser radiation is shifted, on which thus lie the centers of the laser pulses which produce the processing effect in the cornea 5. In order to achieve a circular contour of the lenticle surface 20, in areas of the spiral 32 which lie outside the circular contour the treatment laser radiation is blanked, i.e. modified such, that no processing effects occur there. The connection between the lenticle surface 20 and the flap surface 19 can then be produced by a simple lenticle edge surface 30 in the shape of a circle cone envelope. In the top view 33 of the lenticle surface 20 this is illustrated by a cross-hatched lenticle edge zone 31 which penetrates deeply enough into the cornea for the overall volume 18 to be isolated by the flap surface 19, the lenticle surface 20 and the lenticle edge surface 30.